With the globalization of the elevator industry there has been a trend to standardize elevator systems worldwide. This trend is leaning toward the use of traction systems for smaller elevator applications (i.e., less floors). Previously, hydraulic elevator systems were commonly used in applications with less than seven landings. The trend anticipates that these applications will begin to utilize traction elevator systems. Such systems must be provided with emergency or back-up power systems that supply power not only to the controller, door operator, and valves, but also to the main drive system.
Recent developments have lead to traction elevator systems replacing older technology (i.e., “soft start” systems) with new Variable Voltage/Variable Frequency Drive (VVVFD) technologies. VVVFD technology has two advantages: first, VVVFD technology allows a traction motor to be connected to the main power system with a low level of inrush current; and second, VVVFD technology allows a traction motor to run at both a very low speed and a very low power. Thus, while a typical traction motor might be a 20 hp three-phase load when running at a normal speed, a VVVFD-based motor may only be a 2 hp load at its slowest speed. The reason for this low load is that a traction elevator system comprises a counter-weighted configuration. That is, the elevator's typical loading of passengers (i.e., the passenger weight) is exactly matched by the counter weight. This allows for optimal efficiency of the system. However, under most elevator conditions, an exact matching of the counter weight and the passenger load does not occur. Thus, a traction elevator will tend to drift up or down depending on this imbalance.
By continually monitoring the elevator load, it is possible to keep track of which direction the car would drift. When a power outage occurs, this information is available for use by the emergency back-up power system. Also, to handle the capacitive nature of the VVVFD and its input filtering, a three phase inductor system is placed between an inverter output stage and the VVVFD, so as to compensate for the reactance of the input filter.
Furthermore, unlike the hydraulic elevator systems, a traction elevator system requires that the back-up power system provide full power to the traction motor (e.g., >20 hp load at full motor speed), even when normal building power is present and properly functioning. This requires that the back-up power system be capable of switching a high power load. This requirement to handle high levels of normal power results in a system where the back-up power is fed in parallel to the normal control power system. As a result, it is critical to control the sequencing of the various power systems so as to assure that both the back-up power and the normal control power sources are not simultaneously connected to the traction elevator system.